1. Field of the Invention
This invention relates to apparatus for measuring the initial apparent weight and subsequent changes of a sludge charging a liquid.
In numerous cases of treatment of liquids, particularly of waste water or sewage, it is often necessary to utilise live sludge the evolution of which must be monitored not only from day to day at a given point in the treatment installation, but also at a given moment from one point to another in the installation.
2. Description of the Prior Art
In treatment stations which employ activated sludges, the volume occupied by this sludge after settling for half an hour in a test-tube is measured first in the proximity of treatment basins, and secondly the weight of the dry extract is measured in the laboratory.
After these successive measurements, and with these two concentrations expressed as volume and as weight respectively, the first measurement in millilitres per litre is divided by the second in grams per litre to define a specific volume, which is called the sludge index, quality index, or Mohlman index.
Any inaccuracy or difficulty in either of the terms of the division is reflected in the index.
A first difficulty consists in determining the compacted volume read after settling for half an hour. For a sludge of a given nature this volume is no longer proportional to the weight of the dry extract when the volume or weight concentration exceeds a relatively low limit.
It has already been proposed to predilute the sample of sludge which is to be analysed in such a manner that, after settling for 30 minutes in a 1-litre test tube the compacted volume will be between 150 and 250 millilitres. This presupposes at least one additional preliminary test and there is a consequent waste of time.
A second difficulty arises from the lengthy preparation of the dry extract by centrifuging and stoving before weighing, preceded by any transporting necessary before reaching a laboratory. Knowledge of the weight is generally at best acquired only on the day following the taking of the sample. Moreover, the division operation giving the specific volume is often put off until the next day, and even later in the case of small stations which have no laboratory. Any action required to regulate the installation once the full facts are known is therefore correspondingly delayed.
Moreover, in the case of an increase of the pollution to be treated, the living matter (biomass) of the sludge, which necessarily increases, also causes an increase of the weight of the dry material. Excess sludge has to be drained in an undesirable manner.
Apart from delays of this kind, weighing in the dry state gives rise to other difficulties. It requires nontransportable equipment which is expensive to purchase and to install, because a local laboratory is required, and is expensive to operate because a specialist is needed. In cases where the laboratory is not on the spot it is difficult to coordinate the data from the laboratory, which has knowledge only of the sludge, and data from the treatment station where the circumstances inside and outside the station are known.
Moreover, the sludge reduced to its dry extract does not provide information regarding its weight in the water and its ability to settle, and still less regarding the evolution of this weight of a material, which in reality is a live material, both in the water and in the successive states of oxygenation followed by a lack of oxygen.
In order to facilitate the operation of one or more treatment stations, it is a main object of the present invention to eliminate the difficulties and delays mentioned above, to account for variations in life, temperature, and in the ambient condition, to note the effects thereof; to combine all the measurements in a single place, the treatment station, using the same personnel, and to provide an economical, transportable apparatus which is easy to use even out-of-doors.
A further object of the invention is to enable the operator to intervene in practice at his station within the shortest possible time, such time being further shortened by the fact that in the event of an increase of pollution which is to be treated, the naturally increasing biomass does not necessarily increase the apparent weight and may even reduce it. This avoids undesirable draining operations referred to previously.
The present invention provides apparatus for measuring the apparent weight of sludge charging a liquid, comprising a thermally insulating container, a buoyant vessel shaped to fit into the thermally insulated container for immersion in liquid in the container, and a supporting and reference level probe for the buoyant vessel comprising a ball adapted to engage in an opening in the buoyant vessel and a graduated rod surmounting the ball.